Overrides TextButton.onFocusChanged to use the virtual isFocused field. Individual tabs never receive real Roku focus — JRTabBar manages focus externally via the isFocused field.

Positions the underline indicator inside the button after TextButton sizes itself. The indicator sits below the text at text width, inside the background bounds. When focused, the TextButton border naturally wraps around it (ResumeButton pattern).

Called when isFocused or isButtonSelected changes — updates all visual state

Maps tabTitle to TextButton's text field, triggering TextButton's sizing flow